A typical photodiode array includes a semiconductor substrate of a first conductivity type, having a front side formed with an array of doped regions of a second, opposite conductivity type, and an opposing back side that includes a heavily-doped bias electrode layer of the first conductivity type. For simplicity, the frontside doped regions are referred to below as gates, independent of their function as anodes or cathodes. Similarly, the abbreviation BEL is used to denote the backside bias electrode layer.
To provide a framework for the discussion below, an example of a photodiode array is used below in which the frontside doped gate regions have p-type conductivity, the substrate is n-type, and the backside bias electrode layer is, accordingly, a heavily-doped n-type layer. All statements and claims herein are equally true if the conductivity types of all the layers are reversed and the corresponding changes are made to the polarities of the charge carriers, applied voltages and electric fields.
Typically, the gate and bias electrode layers are formed internally to the crystalline semiconductor substrate. They are therefore native to and homostructural with the substrate. In most implementations, an external gate contact, formed from one or more non-native, heterostructural, conducting layers external to the substrate, is formed over a portion of each of the frontside gates. Similarly, one or more non-native, heterostructural, external back contacts may be formed over all, or a portion of, the backside bias electrode layer. In the case of silicon substrates, the gate contacts are usually formed from one or more metals, metal-silicon intermetallic compounds, or deposited, heavily-doped polysilicon, or a combination of a plurality of these materials. In the present context, polysilicon is considered to be both non-native and heterostructural to the crystalline silicon substrate. Similarly, silicon dioxide (SiO2), the amorphous “native oxide” of silicon, is both non-native and heterostructural to the substrate in this context. Back contacts to silicon photodiode arrays may use the same materials or one or more transparent conducting materials such as indium-tin oxide, which is an amorphous typically non-stoichiometric mixture of indium oxide and tin oxide. In many applications, an array of readout circuits is also formed on the front surface of the substrate.
A potential difference, referred to as a reverse bias, can be applied between the gate and the bias electrode layer to produce a depletion region within the substrate extending into the substrate from the p-n junction between the gate on the front side and the substrate. Hence, a photodiode is effected by the gate, the substrate and the BEL. External gate contacts or back contacts are ancillary elements provided to facilitate electrical connections to the photodiode array, not essential components thereof.
Such a photodiode array may be configured either in a frontside-illuminated mode to receive photons from the front side or in a backside-illuminated mode to receive photons from the backside. The frontside-illuminated mode, however, usually results in a lower external quantum efficiency (ratio of photocarriers collected to incident photons) than the backside-illuminated mode, because the conducting elements of the gate contacts and the readout circuits (if provided) reduce the active photosensitive area of the array on the front side. In comparison, the entire back side can be used to collect incoming radiation when properly configured. All other factors being equal, enhanced photosensitivity results in increased signal-to-noise ratio. In single-particle radiation detection applications using either direct (intrinsic) detection in the substrate or indirect detection (e.g., using scintillators as discussed below), enhanced photosensitivity results in improved particle energy resolution. In addition, conducting lines and other physical features such as steps in dielectric thickness on the front side can scatter light into the photosensitive areas of adjacent photodiodes, thereby reducing image contrast. Contrast degradation modifies the modulation transfer function of the array and can reduce the useful spatial resolution of the array. Therefore, backside illuminated photodiode arrays are frequently used in imaging applications to improve photosensitivity, signal-to-noise ratio, particle energy resolution and spatial resolution.
In a backside-illuminated photodiode, photocurrent is typically generated by band-to-band absorption. Photons with energy greater than the bandgap of the semiconductor substrate enter the back of the substrate and are absorbed, producing electron-hole pairs. If an electron-hole pair is generated outside the depletion region of a gate, the minority carrier (a hole in the example above) may diffuse to the edge of the depletion region beneath one of the gates. The electric field within the depletion region “collects” the hole by accelerating it towards the gate. If, however, a photon is absorbed within the depletion region of a gate, the electric field “collects” the hole as above, but accelerates the electron towards the undepleted substrate, or, if the substrate is fully depleted below a gate, towards the backside bias electrode layer. In either case, the photocurrent will flow through the photodiode and the external circuitry that maintains the bias between the gate and the bias electrode layer. If readout circuitry is provided on same semiconductor substrate, the circuit elements associated with each gate will produce a signal that represents a mathematical function of the photocurrent, the quantity of charge caused by the photon absorption, or a combination of both.
In low light-level imaging applications such as night photography, nuclear medical imaging, photon medical imaging, x-ray computed tomography and ballistic photon detection, it is critical for photodiode arrays simultaneously to exhibit high external photon conversion efficiency (defined as the ratio of photocarriers collected to photons incident on the back surface of the substrate) and extremely low reverse-bias leakage currents. For brevity, “quantum efficiency” is used below to denote external photon conversion efficiency, and “leakage current” is used to denote reverse-bias leakage current.
In low light-level imaging systems, the input optical signal often is in the form of short pulses, a few nanoseconds to a few microseconds in duration. For these applications, it is highly beneficial for the photodiodes to have short pulse response times, often referred to collectively as transition times or, singly, as rise and fall times, in the rough order of magnitude range of 10 to 100 ns or less. The signal processing system associated with the photodiode arrays typically “shapes” the output pulse by integrating the photocurrent generated within a time window of fixed duration in the same order of magnitude as the length of the photopulse.
Photodiode arrays with one or more long transition times produce output photocurrent pulses significantly longer than the input photopulse. These long photocurrent pulses produce smaller output pulses from the signal processing electronics than do those from faster photodiode arrays. Therefore, slow photodiode arrays may may result in low output signals from the signal processing electronics and therefore degrade the signal-to-noise ratio of the image.
To achieve high quantum efficiency, the bias electrode layer should be thin enough to be transparent to the incident radiation, yet sufficiently conductive to provide an equipotential surface on the back surface of the substrate adequate to maintain uniform depletion over the entire area beneath each gate.
The bias electrode layer should also have a low density of crystallographic defects and be free from deep-level impurities. If these criteria are met, the BEL will exhibit long enough minority-carrier lifetimes to minimize recombination of photocarriers generated within the BEL, thereby maximizing the efficiency of collection of photocarriers generated therein The BEL will therefore not be an optical “dead layer.”
The pulse response times of the photodiodes can be minimized by using semiconductor substrates of high resistivity in the approximate range of 5 to 25 kΩ-cm, operated under reverse bias conditions sufficient to fully deplete the substrate under the gates. Under such full-depletion conditions, the electric field of the depletion region extends to the bias electrode layer on the back side.
Achieving ultra-low leakage current densities, e.g., below about 1 nA/cm2 at room temperature, of the individual photodiodes requires reduction of the contributions to the total leakage current by (1) the substrate; (2) the back contact structure (including the bias electrode layer and any additional layers formed thereon); (3) the front surface regions between the gate regions and between the outer gates and the surrounding regions; and (4) the edges of the substrate, formed when the substrate is “diced” to form individual photodiode array chips.
The bulk generation current of the substrate may be reduced by “gettering,” an elevated-temperature process, typically performed at 1000° C. or higher for silicon substrates, in which strained, damaged or heavily-doped layers, singly or in combination, on the back surface of the substrate attract and capture impurities or crystallographic defects. Crystallographic defects may also be annihilated by recombination during the gettering process. The strained, damaged or heavily-doped “gettering layer” or layers is subsequently removed, thereby removing the absorbed impurities and defects. Once the substrate has been gettered, further high-temperature processes should be avoided, to prevent introduction of new impurities or generation of additional defects. All subsequent chemical processing and handling of the substrate should be scrupulously clean to avoid re-contamination.
Front surface generation currents in silicon photodiode arrays are usually minimized by using silicon dioxide (SiO2) layers thermally grown under conditions known to produce low-leakage surfaces. These oxides are typically grown prior to, or concurrently with, the gettering process. Low-temperature (below 400° C.) treatments may be performed after gettering to optimize surface leakage.
Minimizing leakage currents associated with the back contact structure requires dealing with a number of contradictory requirements. As discussed above, the bias electrode layer must be thin enough to be transparent. Such thin layers, however, are easily damaged, and damaged regions may generate leakage current very efficiently. Under fully depleted conditions, the bias electrode layer must be contain enough electrically active (i.e., charged as opposed to neutral) dopant atoms to allow it to terminate the electric field of the depletion region. The back surface of the crystalline semiconductor substrate, or the interface between the substrate and overlayers formed upon the back surface of the substrate to enhance the backside conductivity or to reduce reflection of incident photons, is a region where the crystal structure of the substrate is imperfectly terminated. Such regions may be capable of generating high leakage currents. If the electric field penetrates the BEL and reaches such an interface, it will efficiently collect the current generated there. Similarly, the electric field will collect leakage current generated by impurities or defects within the BEL. Undepleted regions in the BEL typically are highly-doped and will not be efficient leakage generators. Deep level impurities and defects in depleted regions of the BEL, on the other hand, may generate large leakage currents. If the BEL does not exhibit long minority carrier lifetimes, the BEL may therefore degrade the leakage current of the photodiodes.
Edge leakage currents are usually suppressed by providing guard structures surrounding the array of pixels. These structures collect the leakage currents generated at the diced edges before it reaches the pixels themselves.
Guard structures have been developed for a wide variety of applications. In particular, many configurations of guard structures—often referred to as field terminations or, simply, terminations—have been developed to suppress surface breakdown in power semiconductor devices. These terminations typically consist of a series of annular, heavily-doped “field rings”, also referred to as guard rings. Frequently, one or more of the field rings is electrically connected to annular, conducting layers overlapping insulating regions adjacent to the guard ring, to control the surface potential of the semiconductor substrate. Such annular conductors are referred to as equipotential rings. Typically, the field rings and associated equipotential rings are passive, floating devices, but some embodiments employ active, biased elements. Biased, conducting layers insulated from the semiconductor substrate are referred to as field plates.
The primary purpose of field termination structures is to control the distribution of the electric field parallel to the front surface of the semiconductor substrate. For brevity, the term horizontal is used below to denote this direction. A well-designed termination generates a relatively uniform horizontal electric field configuration in the spaces between the guard rings. In particular, the termination should reduce the maximum values of the horizontal electric field, thereby suppressing surface breakdown and surface leakage currents.
Other methods of surface breakdown suppression include beveling of the front surface, diced edges, or both; trench etching; surface doping profile modification by ion implantation; and formation of semi-insulating surface regions by ion implantation damage in wide bandgap substrates (e.g., gallium arsenide [GaAs]).
To minimize surface leakage currents, the guard structure for a low leakage current semiconductor photodiode array should be designed in accordance with the same principles as those for power devices. Except for photodiode arrays specifically designed to employ avalanche mechanism as a gain mechanism, low noise photodiode arrays seldom operate at voltages near those that would result in bulk dielectric breakdown as the substrate. As discussed below, however, reducing the size of the guard structure in a photodiode array may result in high horizontal electric fields.
In both power semiconductor devices and low-noise semiconductor photodiode arrays, the field termination structure is ancillary to the primary use of the device. The presence of the guard structure increases the area of the individual dice [“chips”] and therefore reduces the efficiency of utilization of the surface area of the substrate for the primary purpose. In photodiode arrays designed to be assembled, or “abutted”, into a mosaic detector comprised of a plurality of photodiode array chips, the guard structure reduces the effective photosensitive area of the mosaic detector. They also introduce optically inactive gaps between the outer photodiodes in adjacent arrays. Unless the spacing of the individual photodiodes in an individual array are purposely positioned far enough apart to match the spacing imposed by the guard structure—thereby further reducing the areal utilization efficiency—the presence of the guard structure causes spacing of the pixels in the mosaic detector to be non-uniform. Non-uniform pixel spacing complicates image analysis and detracts from the image quality of the detector.
Therefore, in semiconductor photodiode arrays, it is highly desirable to minimize the area occupied by the guard structure. Despite the low electric fields present in the depletion regions of the individual photodiodes, the guard structure must be designed carefully to avoid high electric fields and the associated high surface leakage currents.
The magnitude of the edge generation currents injected into the semiconductor chip must be considered in the design of the guard structure for the photodiode array. All other factors being equal, larger edge generation currents result in larger currents collected by the guard structure. For brevity, the current collected by the guard structure is referred to below as the internal guard current. Typically, some fraction of the internal guard current may flow through circuit elements common to the current paths of the guard structure and the individual photodiode pixels. These circuit elements may be parasitic elements internal to the photodiode array chip, parasitic elements associated with contacts and wires in the external circuitry, intentional circuit components, or combinations of a plurality of elements of any of these types. For brevity, the term “guard current” is used below to denote the portion of the internal guard current flowing through these common elements. The noise generated in the common elements by the guard current increases monotonically with the guard current—frequently proportionally to the square root of the guard current. Ceteris paribus, larger edge generation currents result in a higher noise level in the image and lower image quality.
A single guard structure may incorporate one or more passive regions that suppress edge currents by, for example, recombination of minority carriers, and one or more active regions that collect minority carriers injected from the diced edge. In such structures, the internal guard current collected by an active region may be reduced by increasing the distance between the active region and the diced edge. All other factors being equal, in totally passive guard structures, reduction in the proportion of the edge generation current collected by the outer photodiode pixels in the array can be accomplished only by increasing the distance between the outer pixels and the edge of the chip. Increasing the distance of the active guard regions or the outer pixels from the diced edge increases the area occupied by the guard structure, with the associated deleterious effects discussed above.
Therefore, what is needed is a method to reduce the gross generation current injected into the substrate by the edges of the photodiode array chip, thereby facilitating a reduction in the area occupied by the guard structure, with the associated performance benefits implied by the preceding discussion.